1. Field of the Invention
This invention relates generally to a rotary-head, helical-scan video tape recorder that records a video signal and an audio signal in the same slant track and, more particularly, to a video tape recorder for editing the video and audio signals separately to perform after recording.
2. Description of the Background
A video tape recorder (VTR) is known in which an audio signal is recorded in the same slant tracks as the corresponding video signal. In such VTR a pair of heads are oppositely arranged at 180.degree., and the tape is wrapped at a wrap angle greater than 180.degree. for instance, about 210.degree.. The angle range of 180.degree. is used as the recording area for the video signal, and the overlap angle range of about 30.degree. is used as the recording area for a time-base compressed digital audio signal. This kind of VTR is also used to perform the so-called after recording, in which either the audio or video signal that has been recorded is changed or edited. Such after recording involves playing back the video portion, for example, while recording a new audio signal in place of the audio signal that was originally associated with that video signal.
In such case, when after recording a video signal or an audio signal, a recording voltage is ordinarily present at the recording head of around 0.5 to 2 V.sub.p-p. On the other hand, upon playback, a reproducing voltage of only 0.1 mV to a few mV is generated at the head. It is then seen that the recording voltage is much, much larger than the reproducing voltage at the head. Thus, because in after recording the recording signal is intermixed with the reproduction signal, the reproduced image or reproduced audio signal is deteriorated to the point that it is lost. Therefore, it is very difficult to after record a video image while listening to the reproduced sound. On the other hand, in the case of after recording an audio signal while observing the reproduced image, the reproduced image is also disturbed during the audio recording time by this disparity in signal voltage levels.
Accordingly, a process to replace the reproduced video signal by another video signal in this period is needed. One method that has been proposed to cope with such a problem involves providing a reproducing amplifier in the head drum, so that the reproduced output is amplified to the same level as the recording voltage at a point very near the head. In another proposed solution a separate head is provided that is used only for after recording and any possible recording and reproduction time overlap is prevented by using a memory.
Both of these proposed solutions suffer drawbacks. For example, in the construction in which the reproducing amplifier is provided in the drum, it is necessary to supply power to the amplifier, so that a slip ring or a rotary transformer must be used. Slip rings, however, have a problem in terms of reliability. In the case of the rotary transformer, such transformers are generally expensive and, also, to prevent the mixing of the signals in the transformer, the frequency range of the signal is necessarily limited. On the other hand, practicing the latter method also has disadvantages, because a special head for after recording must be provided.